Conventional plasma etching of dielectric structures on workpieces has issues of reactive ion etch (RIE) lag (e.g., aspect ratio dependent etching), etch stop, or twisting of features etched. This is due in part to both competing polymer deposition mechanism using a fluorocarbon etch chemistry, and positive charging of the structure walls from predominantly positive ion fluxes and unequal electron fluxes down the depth of the structure. In addition, this charging leads to selectivity and damage issues, especially to the soft films encountered in dielectric etching. For example, some materials, such as insulating materials (e.g., dielectric materials) can charge during plasma etch processing. The charging of the material can result in damage, destruction, or over-etching of the material due to, for example, increased flux of a plasma species at a charged surface of the material.
Solutions to the surface charging problem include, for example, pulsing the RF source and bias frequencies to dissipate charge from the surface of the material. Typically, the pulsing of the RF source and bias signals is synchronized such that each RF signal is completely aligned, e.g., each signal is in phase and has the same duty cycle. Unfortunately, this type of synchronization does not effectively compensate for variations in the plasma due to the pulsing process which can result in a non-uniform plasma and a poorly controlled etch rate.